Insect communities in soybeans of eastern South Dakota: The effects of vegetation management and pesticides on soybean aphids, bean leaf beetles, and their natural enemies

Authors: Lundgren, Jonathan; Hesler, Louis; CLAY, SHARON - South Dakota State University; FAUSTI, SCOTT - South Dakota State University

Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/9/2012
Publication Date: 11/28/2012
Citation: Lundgren, J.G., Hesler, L.S., Clay, S.A., Fausti, S.F. 2013. Insect communities in soybeans of eastern South Dakota: The effects of vegetation management and pesticides on soybean aphids, bean leaf beetles, and their natural enemies. Crop Protection. 43:104-118.

Interpretive Summary: Insect communities in soybeans have changed dramatically since the invasion of the soybean aphid in North America approximately 12 years ago, but the regional insect community toward the western front of soybean production has never been thoroughly described. Insect communities are tied to plant species assemblages, and thus weed management practices have important implications for which insects are present within soybeans and for outbreaks of the soybean aphid. We studied how three soybean production systems that varied in the intensity on which they rely on pesticides affected insect and weed communities in South Dakota soybeans. Weeds communities were reduced the most in the chemically intensive treatment that relied most heavily on herbicides, but a reduced chemical treatment that used only one application of herbicide performed equally well in spite of slightly higher weed densities. Insecticides effectively eliminated soybean aphids, typically with one application, and bean leaf beetles (a sporadic pest of soybeans in this region). Predators of soybean aphids that lived in the plant canopy were virtually eliminated by the insecticide application, and populations did not recover during the rest of the season. Soil dwelling predators were unaffected by insecticides. A cover crop treatment with minimal pesticide use had low weed populations when the cover crop was eliminated from the field, and very high insect diversity compared with the chemically based management systems. But aphids often reached high levels in this treatment. The chemically intensive treatment had the highest profitability of the three systems tested, although input costs were lowest in the cover crop system. Optimizing this cover crop system to increasing yields and possibly taking advantage of organic premiums would make this system superior both economically and environmentally. Clearly, weed and insect management decisions in soybeans are inter-related, and we advocate using this type of systems-level approach for investigating the complex interplay between farm management practices.

Technical Abstract: Although most pests of soybeans in the Northern Great Plains are managed using pesticides, farm management practices that encourage biodiversity offer promising long-term, sustainable solutions for controlling insect and weed pests profitably. The recent invasion of the Northern Great Plains by the soybean aphid (Aphis glycines; Hemiptera: Aphididae) has had potentially important implications for insect communities in soybeans of this region, although recent descriptions of this regional community are scarce. Here we describe how three pest management systems that vary in the intensity with which they rely on herbicides and insecticides (chemically intensive, reduced chemical, and spring cover crop treatments) affect insect pest populations, arthropod predator communities, weed assemblages, and soybean yield and profitability. Soybean aphids exceeded economic thresholds in all three years, and insecticides successfully suppressed these outbreaks in the two chemical treatments; aphids exceeded the economic injury level in the cover crop treatment in 2 of 3 study years. Bean leaf beetle (Certoma trifurcata; Coleoptera: Chrysomelidae) populations were sub-economic in all treatments; insecticides targeting soybean aphid also reduced bean leaf beetles in the first year of study when beetle populations were at their highest. Foliar-dwelling predator populations were substantially higher in the cover crop treatment than the chemical treatments in all years of study; population declines in the latter treatments were strongly associated with insecticide applications targeting soybean aphids. Foliar predator populations did not rebound within the growing season after insecticides were applied. Soil predator populations were largely unaffected by treatment (except in 2006, when they were more abundant in the cover crop treatment than the chemical treatments). Weed communities varied among treatments and study years, with few consistent trends except that the chemically intensive treatment had lower weed densities than the other treatments. Although input costs of the cover crop and reduced chemical treatments were lower than the chemically intensive treatment, the chemically intensive treatment was the most profitable of the three. Nevertheless, we contend that the cover crops can be managed more efficiently in order to increase the profitability and competitiveness of this treatment while gaining the long-term benefits gleaned from conserving biodiversity in our agroecosystems.